Polymeric heat exchangers are commonly used in a variety of applications and are made using a variety of manufacturing techniques. Each of these techniques tends to produce products having a unique set of attributes that render them to be either more or less effective relative to any given specific application. Common to them all is the desire for exceptionally strong, leak proof joints.
One such common application is their use as solar collectors for heating swimming pools. This type of heat exchanger is predominantly made of a thermoplastic polypropylene copolymer, especially stabilized for outdoor usage. Some are also made of a thermoset ethylene-propylene diene based polymer.
Solar collectors basically comprise a plurality of hollow conduits in a coplanar orientation joined between hollow manifolds that are located at each of the hollow conduit ends. The hollow conduits are made by extrusion processing, and are preferably, though not always, made in a circular cross-section or tubular form. Common constructions include closely spaced individual tubes and tube mats that comprise a plurality of tubes joined together, either directly side to side, or through an interconnecting web. Depending upon the manufacturing technique used, the manifold inlets to the tubes of common collector constructions can be either fully open relative to internal tube size, or in some way constricted.
To solar heat in-ground swimming pools, a constricted inlet is desirable. This is because all solar heating systems for in-ground pools require a multiplicity of collectors installed manifold end to manifold end. As the number of collectors in such a continuous array increases, water flow through the last collector in the array relative to the first becomes more and more reduced with increasing length of the array. Those constructions having fully open inlets to each and every tube directly off the manifold start to have performance impairing flow uniformity problems once the array goes beyond a very few collectors. Those constructions having some form of flow constriction achieve superior flow uniformity throughout the array and consequently superior heating performance. Some constructions have constrictions positioned at each and every tube end. This approach has been found to be highly effective for array lengths typically used in residential pool systems.
Other solar collector constructions constrict flow by including a subplenum. A subplenum is a separate chamber secondary to the main manifold chamber. On the tube side of this secondary chamber are a multiplicity of openings that communicate with each and every tube inlet. On the manifold side are more widely spaced and fewer openings. Because the main manifold has fewer openings, the flow reduction along an extended array can be significantly reduced. The subplenum serves as a distribution chamber, uniformly distributing water to the tubes, itself being uniformly fed by the main manifold.
The subplenum technique is capable of achieving the least amount of flow reduction, which is especially beneficial in very long arrays. It also creates excessive back pressure if the array length is short relative to the degree of manifold constriction designed into the collector. Since the very long arrays necessary to adequately realize the advantage of the subplenum technique are rarely encountered in solar heating residential swimming pools, the subplenum technique does not provide any real value in such an application.
Therefore, it can be seen that both the fully open inlet and the subplenum type constricted inlet are not so much a part of the construction of common residential swimming pool heating collectors because of what they bring to the application. They are there because they are a necessary part of the construction technique used to make the collector. However, applications exist where each of these inlet configurations individually excels. The fully open inlet construction excels whenever a thermally induced siphon flow is desired. The subplenum type constricted inlet excels whenever extremely long arrays are desired. Ideally the inlet configuration should match the application and not be dictated by the manufacturing technique.
For heat exchangers not involving solar energy, it is often desirable to have a multiplicity of planes of coplanar individual tubes or tube mats that are joined into common manifolds. This serves to greatly expand the effective heat exchange area serviced by the common manifolds. Such constructions are useful for a great many types of heating and cooling applications, wherein the non-corrosive characteristics of polymeric materials are desired. Common polymeric thermoplastic materials of construction include polypropylene, polyvinylidene fluoride (PVDF), and copolymers of polytetrafluoroethylene(FEP, PFA, ETFE, ECTFE, etc).
Whatever the application, the integrity of the joint between the hollow conduit ends and the hollow manifold of a heat exchanger is of key concern, as is the cost to make it. In the generic overmolding method for making this joint, a plurality of thin wall hollow conduits, which are internally supported by mandrels, are clamped into a mold. Molten plastic fills a hollow cavity surrounding the hollow conduits to form a common hollow manifold around the hollow conduits. The integrity of the joint depends upon the degree of bonding obtained at the interface between the manifold and the conduit.
U.S. Pat. No. 4,352,772 describes such a generic overmolding process in which hollow conduit supporting mandrels extend from an internal manifold core around which the manifold forms. The supporting mandrels and the manifold-forming core are removed at the end of the molding cycle. The process only makes fully open, non-constrictive tube inlets, which are not desirable for the extended array collector systems common to in-ground pool heating needs. Furthermore, the stated objective was to obtain a joint having a mechanical strength equal to the strength of the base materials. This disclosure acknowledges the use of high temperatures and pressures to obtain it, although it does not identify what they are. Through years of observation of the field experience of solar collectors made using this technique, it is apparent that such a bond is neither equal to the strength of the base materials nor sufficiently strong for the application. Indeed, two separate manufacturers using this generic overmolding method have both resorted to enlarging the open internal mold cavity in the area surrounding the hollow conduits in an apparent attempt to reinforce the strength of a joint which had proved itself to be inadequate during initial usage. Experimentation with this method revealed that polypropylene copolymer molded at normal injection molding melt temperatures of 450–480 degrees F. achieved no bonding. It was found that melt temperature is the key variable and had to be in excess of 500 degrees F. to observe what even can begin to be called a bond to start to form. It is known to those skilled it the art, that such high melt temperatures deteriorate the toughness and weld line integrity of molded articles, as well as makes it more difficult to limit flashing. The weld line integrity is a particularly important issue. Within the normal range of molding melt temperatures, weld line integrity increases with increasing melt temperature. However, in going above the normal range, decomposition of the molecular chains predominates, and weld line integrity decreases with further increasing melt temperature at a rapidly accelerating rate. The generally accepted rule of thumb is that the rate of decomposition doubles, with each 10 degree C. (18 degree F.) incremental increase in temperature. One needs only to understand that decomposition begins to take place at temperatures far lower than normal injection molding melt temperatures, to quickly appreciate the degree of rapid acceleration on decomposition rate that had already been steadfastly doubling to that point.
U.S. Pat. No. 4,740,344 discloses an overmolding process in which the hollow conduits, supported by removable mandrels, are inserted into openings along the floor of an insert. This assembly is clamped within a mold, the tube ends are preheated to an elevated temperature below the melting temperature of the tube material, and molten plastic is injected into a cavity formed by the mold, the insert, and the tube ends. The initially elevated tube temperature boosts the ability of the molding melt to more greatly reduce tube surface viscosity, thus improving spreading and improving conformance to the overmolded molten plastic surface, and hence providing superior bonding. Unfortunately, the tube preheating operation is counterproductive to high volume production of polymeric commercially viable solar collectors.
An object of the present invention is to provide a heat exchanger having an improved bond between its manifold and its hollow conduits due to increased interfacial contact between the hollow conduits and the overmolding plastic that forms the manifold.
Another object of the present invention is to provide an overmolding process that achieves an improved bond without resorting to higher than normal injection molding temperatures or preheating the inserted conduits.
Another object of the present invention is to provide a heat exchanger with manifold-to-conduit inlets that can be selectively configured either fully open or partially constricted to achieve optimum performance in either pumped or thermally induced siphoned flow systems.
Another object of the present invention is to provide a heat exchanger with manifold-to-conduit inlets that can be selectively constricted either at each and every tube inlet or using the subplenum technique, or both.
An advantage of the heat exchanger of the present invention is that it is better able to withstand the thermal and mechanical stress exerted upon it during operation than known heat exchangers.
Another advantage of the heat exchanger of the present invention is that it adds an additional highly developed thermally fused seal within the interior chamber of the manifold that completely encapsulates the area where the hollow conduits penetrate through the manifold wall.
An advantage of the overmolding process of the present invention is that it permits efficient, low cost manufacturing of solar collectors having improved, water-tight interfacial bonds between the hollow conduits and the manifold wall.
Another advantage of the overmolding process of the present invention is that it permits efficient, low cost manufacturing of heat exchangers, whereby the hollow conduits may be positioned on multiple planes, extending the effective heat transfer area serviced by the manifolds.
Another advantage of the overmolding process of the present invention is that it is adaptable to low cost manufacturing of polymeric heat exchangers that can be selectively configured to suit a wide variety of applications.